Add $mq-static-breakpoint to control queries for static stylesheets

Instead of simply outputting all min-width queries, we can now select a
target breakpoint, and only output queries that span (or start at) that
width.

This allows us to have breakpoints that are higher than our target
width for older browsers (such as a 'widescreen' breakpoint)
without serving those min-width queries in our static/rasterized output.